The long-term objective is to improve understanding of cell cycle control in eukaryotic organisms. The experiments will be carried out with the fission yeast Schizosaccharomyces pombe. Fission yeast has proven to be a very useful organism for studies aimed at discovering basic features of cell cycle control that are conserved amongst all eukaryotic organisms, including humans. The studies will provide important insights into the general properties of cell cycle control and offer a valuable framework for the investigation of more complex multicellular organisms. Elucidation of cell cycle controls will greatly aid the rational investigation and treatment of human diseases related to cell proliferation abnormalities. The investigations will focus on the control mechanisms which function in the negative regulation of Cdc2. Central to this control are the Wee1 and Mik1 tyrosine kinases, which inhibit the activity of Cdc2/cyclin-B by phosphorylating Cdc2 on tyrosine-l5. The specific aims of the project are: 1. To explore the regulation of Wee1 and Mik1. It has been proposed that amplification of Cdc2/cyclin-B kinase activity during G2/M is accelerated by a positive feedback loop in which Cdc2/cyclin-B inhibits Wee1 kinase by direct phosphorylation. The existence and importance of such a control circuit will be determined in genetic and biochemical experiments. It will also be determined which Cdc2/cyclin-B kinases are regulated by Wee1 and Mik1. 2. To discover the strategic role of Nim1 kinase. Nim1 functions as a mitotic inducer by inhibiting Wee1. The strategic role of Nim1 in the mitotic control will be explored. 3. To understand how the mitotic control responds to cellular stress. Recent experiments have uncovered a complex regulatory network which is used to modulate the mitotic control when cells encounter physiological stress. Details of the operation of this network will be determined.